1. Field of the Invention
The present invention relates generally to thermoelectrochemical converters useful for converting thermal energy into electrical energy. More particularly, the present invention involves electrochemical cells wherein a Lewis acid and Lewis base react to produce electrical potential and a Lewis salt which is then thermally decomposed to regenerate the acid and base.
2. Description of Related Art
Thermally regenerated electrochemical systems have been investigated extensively since the late 1950's. In these systems, the working substances utilized in an electrochemical cell to generate electrical current are regenerated by providing thermal energy to the products of the electrochemical reactions. Typically, these products decompose to reform the working substances, or the elevated temperature provides heat for distillation columns to separate liquid components from solid components. These systems are similar to secondary batteries in many respects except that regeneration of the electrochemically active reactants is accomplished thermally rather than electrically.
Thermally regenerated fuel cell systems which are based upon the oxidation and reduction of hydrogen have gained wide acceptance because hydrogen electrodes are very efficient. Such electrodes are capable of supporting reasonably high current flow and they are well known in the industry. Additionally, the low activation and low mass low transfer losses of these electrodes contribute substantially to overall system efficiency. Representative fuel cells which utilize hydrogen electrodes are described in U.S. Pat. No. 4,738,904 which is assigned to the same assignee as the present invention.
Among other systems which are described in U.S. Pat. No. 4,738,904 is a fuel cell using oxygen electrodes and having a cathode electrolyte of concentrated sulfuric acid and an anode electrolyte of dilute sulfuric acid. The difference in acid concentration between the two solutions is maintained by heating the concentrated solution to distill off water generated at the cathode. A disadvantage associated with thermoelectrochemical systems that use the distillation process is that they require bulky equipment and the need to circulate large amounts of water. Additionally, aqueous based systems require an external system for returning hydrogen from the cathode to the anode. This external return system makes the fuel cell more complex and is prone to leaks. A preferred method for transferring the hydrogen from the cathode to the anode is through a porous cell separator. However, the low surface tension in aqueous systems allows electrolyte to flood the pores of these porous separators.
Another system described in U.S. Pat. No. 4,738,904 is an improvement to the fuel cell described immediately above, in which a buffered solution containing sodium sulfate and sodium bisulfate is substituted for the dilute acid. During operation, sodium bisulfate is generated at the anode and sodium sulfate is consumed. For regeneration, the sodium bisulfate is thermally converted to sodium sulfate, water, and sulfur trioxide. The sulfur trioxide is combined with water to regenerate sulfuric acid. This system has the advantage of generating electrical energy without requiring distillation. However, this system does require hydrogen gas containment and transfer lines.
The thermally regenerated fuel cell disclosed in U.S. Pat. No. 4,738,904 utilizes a fluid Bronsted acid and a fluid Bronsted base in the cathode and anode respectively. The anion of the acid combines with the cation of the base to form a salt which is thermally regenerated at temperatures below 250.degree. C. This system, however, has high electrolyte resistance and for most applications inert solvents are required.
Although the above-described fuel cells are well suited for their intended uses, there is a continuing need to provide thermoelectrochemical converters having solventless reactants.
There is also a continuing need to provide thermoelectrochemical converters which do not require the removal of inert solvents by energy-consuming fractional distillation techniques.
There is further a continuing need to provide thermoelectrochemical converters which avoid the need for hydrogen gas containment reservoirs and hydrogen gas transportation and return systems.
There is additionally a continuing need for high efficiency thermoelectrochemical converters which are capable of thermally regenerating the working electrolytes and which produce electrical energy from the waste heat of an internal combustion engine.
There is also a need to provide thermoelectrochemical converters with low internal resistance and high electrolyte conductivity.